Air bearings

ABSTRACT

A journal gas bearing assembly comprising a shaft and a bearing sleeve rotatable one relative to the other. Longitudinal movement of the shaft and 1 bearing sleeve relative to each other in the direction is limited by a hydrostatic thrust bearing formed by a cushion of gas trapped under pressure in a closed volume defined between the shaft and the bearing sleeve. Relative longitudinal movement between the shaft and the bearing sleeve in the reverse direction may be limited by providing for venting the closed volume at the beginning of relative longitudinal movement between the shaft and the bearing sleeve in the reverse direction or by providing a hydrodynamic thrust bearing between the shaft and the bearing sleeve acting in the opposite sense to the hydrostatic thrust bearing.

United States Patent 11 1 Williams AIR BEARINGS [75] Inventor: RaymondWilliams, Appleton,

England [73] Assignee: United Kingdom Atomic Energy 1 Authority, London,England 22 Filed: Mar. 9, 1971 21 Appl.No.: 122,491

[30] Foreign Application Priority Data 3,503,658 3/l970 Remmers 308/.001

OTHER PUBLICATIONS lBM Technical Disclosure Bulletin Received in P0.

Primary Examiner-Charles J. Myhre Assistant ExaminerFrank SuskoAtt0rney-Larson, Taylor and Hinds 57 ABSTRACT A journal gas bearingassembly comprising a shaft and a bearing sleeve rotatable one relativeto the other. Longitudinal movement of the shaft and 1 bearing sleeverelative to each other in the direction is limited by a hydrostaticthrust bearing formed by a cushion of gas trapped under pressure in aclosed volume defined between the shaft and the bearing sleeve. Relativelongitudinal movement between the shaft and the bearing sleeve in thereverse direction may be limited by providing for venting the closedvolume at the beginning of relative longitudinal movement between theshaft and the bearing sleeve in the reverse direction or by providing ahydrodynamic thrust bearing between the shaft and the bearing sleeveacting in the opposite sense to the hydrostatic thrust bearing.

31 Claims, 9 Drawing Figures PAIENTEnsEPnma v 3-758'1.

' sumanra glil 38 III/II! AIR BEARINGS BACKGROUND OF THE INVENTION Thisinvention relates to journal and bearing assemblies which operate withgas lubrication between the journal member and the bearing member.

A typical form of gas bearing assembly of the type referred to abovecomprises a shaft and a co-operating bearing sleeve, the surface of theshaft and the hereof the bearing sleeve being finished to an extremelyhigh standard of accuracy and smoothness to provide bearing surfacesbetween which gas lubrication can be maintained under normal operatingconditions.

There are two main types of journal gas bearing assembly.

In the first type, which is known as a hydrostatic pressure fed journalgas bearing, gas lubrication is maintained between the bearing surfacesof theshaft and the bearing sleeve by gas which is fed under pressurefrom an external source to the interspace defined beween. the bearingsurfaces.

In the second type, which is known as a hydrodynamic self-acting gasjournal bearing, gas lubrication is maintained between the bearingsurfaces of the shaft and the bearing sleeve by the pressure generatedhydrodynamically in, gas in the interspace defined between the bearingsurfaces due to relative rotation of the shaft and the bearing sleeve.

Gas bearing assembliesof the kind referred to above can be maderelatively cheaply, as disclosed in British Patent No. 979,731, byforming at least one of the bearing surfaces of the shaft and thebearing sleeve from a moulded plastic material such asan epoxy resin.

Both types of gas bearing assembly referred to above can be operated.with the, bearing sleeve rotatable on a stationary shaft oralternatively the shaft may be rotatable in a stationarybeari'ng sleeve.In either casev it is. necessary to. support the rotating member againstlongitudinal movement on the stationary member.

SUMMARY OF THE INVENTION the bearing sleeve one relative to the other inone direction is limited by a hydrostatic thrust bearing formed by acushion of gas trapped under pressure in a closed volume defined betweenthe shaft and the bearing sleeve.

The provision of a hydrostatic thrust bearing in a gas journal bearingassembly as specified above only limits longitudinal movement of thebearing sleeve and the shaft relative one to the other in the onedirection. Although such unidirectional longitudinal support issufficient for many applications there is a requirement in some casesfor the shaft and the bearing sleeve to be limited in longitudinalmovement relative to each other in both directions so that preciselongitudinal location of the rotating member of the bearing assembly ismaintained during operation particularly at high speeds.

According to a further aspect of the present invention in a journal gasbearing assembly of the kind specified means are provided for limitinglongitudinal movement of the shaft and the bearing sleeve one relativeto the other in the reverse direction to the direction in which saidhydrostatic thrust bearing limits relative longitudinal movement betweenthe shaft and the bearing sleeve. For example means may be provided forventing the cushion of gas in the hydrostatic thrust bearing at thebeginning of relative longitudinal movement between the shaft and thebearing sleeve in the reverse direction. Alternatively a hydrodynamicthrust bearing may be provided acting between the shaft and the bearingsleeve in the opposite sense to which the hydrostatic thrust bearingacts between the shaft and the.

bearing sleeve. In this case also, inv order to reduce manufacturingcosts, one or both of the bearing surfaces of the hydrodynamic gasbearing may be formed by a moulded plastic material.

Means may be provided for feeding compressed gas to'the hydrostatic gasbearing, for instance from an external source of compressed gas.Alternatively compressed gas may be supplied from the. interspacebetween the bearing surfaces of the shaft and the bearing sleeve to thehydrostaitc thrust bearing.

DESCRIPTION or THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENTSReferring to FIG. 1 of the drawings a bearing assembly l is shown'inwhich a hard faced shaft 2 is supported by a flexible mounting 3 in asupport member 44. The shaft is surrounded by a close fitting bearingsleeve member 5 which is rotatable on the shaft 2. The bearing sleeve 5has a blind ended bore which provides a trapped volume 7 between theblind end of the bearing sleeve 5 and the end of the shaft 2. The shaft2 has a precision ground outer surface and the bore of the bearingsleeve 5 has a plastic lining 8 formed with a surface of gas bearingquality complementary to the surface of the shaft 2. The bearing surfacein the plastic lining 8 of the bearing sleeve 5 may be formed forexample by the method disclosed in British Patent No. 979,73 l.

The shaft 2 hasa longitudinal drilling 9 which is sealed at the end ofthe shaft 2 inside the trapped volume 7 by a ball end stop 10. A feedjet 1] of small diameter than the-drilling 9 connets the drilling 9,from just below the ball end stop 10, with the trapped volume 7.Adjacent the blind end of the bearing sleeve 5 there are twodiametrically opposed vent ports 12.

The flexible mounting 3 comprises a ring 13 of soft rubber bondedbetween outer and inner metal sleeves l4 and 15. The outer metal sleeve15 is an interference fit in a housing 16 in thesupport member 4. Theshaft 2 has an end part 17 of reduced diameter which fits the innermetal sleeve 14 of the flexible mounting 3 and is threaded to fit aretaining nut 18.

In operation of the bearing assembly shown in FIG. 1 of the drawings thebearing sleeve 5 is driven on the shaft 2 by a friction drive belt 19which engages the bearing sleeve tangentially. The bearing assembly 1operates as a hydrodynamic self acting gas journal bearing, transversejournal loads on the bearing sleeve 5 being supported by the pressuregenerated hydrodynamically in the air film in the interspace between thesurface of the shaft 2 and the bearing surface in the plastic lining ofthe bearing sleeve 5.

Compressed air is fed into the trapped volume 7 at the blind end of thebearing sleeve 5 through the drilling 9 in the shaft 2. The pressurisedcushion of air set up in the trapped volume 7 provides a hydrostaticthrust bearing supporting the bearing sleeve 5 longitudinally on theshaft 2. Under normal end loading conditions in the downwards directionthe bearing sleeve 5 runs on the shaft 2 in the position shown in FIG.1, that is with the vent ports just covered by the end of the shaft 2.The compressed air is fed through the drilling 9 in the shaft 2 at apressure sufficient to support the bearing sleeve 5 against normaldownwards end loading on the shaft 2. Transient variations in endloading acting on the bearing sleeve 5 are accommodated by longitudinalmovement of the sleeve 5 on the shaft 2. For example a transientadditional downwards end loading on the bearing sleeve 5 forces thebearing sleeve 5 down on the shaft 2. Because the ports 12 in thebearing sleeve 5 are covered by the end of the shaft 2 and because thefeed jet 1 l is a restricted cross section the additional downwards endloading acting on the bearing sleeve 5 is resisted by furthercompression of the air in the trapped volume 7. On return to normal endloading conditions the bearing sleeve 5 returns to its normaloperational attitude on the shaft 2. On the other hand if the endloading acting downwardly on the bearing sleeve 5 reduces from normalthe bearing sleeve 5 will lift on the shaft 2 so that the vent ports 12in the bearing sleeve 5 are uncovered by the end of the shaft 2.Therefore the pressure of the cushion of gas in the trapped volume 7falls to atmospheric pressure and the sleeve 5 drops back to assume itsnormal operational attitude on the shaft 2 when normal downwards endloading of the bearing sleeve 5 results.

Hydrodynamic self-acting journal gas bearings generally have 'a limitingspeed of operation because of the difficulty of ensuring dynamicstability. The problem arises from the fact that the load carryingcapacity of such bearing reaches a limiting value with increasing speed.In one form of instability the rotating member is subject to resonantoscillations at a critical speed which can result in damaging contactoccuring between the bearing surfaces. One of the most serious forms ofinstability is the so called half speed whirl" which results from thenatural disposition of the rotating member of the bearing assembly toprecess about a fixed centre at approximately the means rotational speedof the air film between the bearing surfaces and which is excited to asufficiently large amplitude to cause extensive damage by contactbetween the bearing surfaces when the precessional speed coincides witha natural resonance frequency of the rotating member of the bearingassembly. I

The bearing assembly of FIG. 1 has several features which are of generalsignificance in raising the speed at which such self acting hydrodynamicgas journal bearings can be operated before the onset of operationalinstability occurs.

A self acting hydrodynamic gas journal bearing may be rendered stable athigher operating speeds by subjecting the rotating member to a sideloading so that it runs eccentrically with respect to the stationarymember. In the bearing assembly of FIG. 1 the friction drive belt 19applies such a side loading on the bearing sleeve 5. Also the frictiondrive belt 19 engages with the bearing sleeve 5 within the effectivebearing length. Thus the transverse loading applied by the drive belt 19on the bearing sleeve 5 is fully supported by the drive belt 19 on thebearing sleeve 5 is fully supported by the bearing and this systemimposes the minimum transverse couple on the bearing sleeve 5 which isanother factor contributing to stability of operation of the bearing athigh speeds.

The mounting of the shaft 2 from one end by the flexible mounting 3pennits three damped degrees of freedom of movement of the shaft 2.

In the first two degrees of freedom of movement of the shaft 2 it canoscillate with the bearing sleeve 5 in a cylindrical mode or in aconical mode about the rotational axis of the bearing sleeve 5. Thesemodes of freedom of movement of the shaft 2 result in the damping out ofdamaging resonant oscillations of the bearing sleeve 5 thereforeallowing the bearing assembly to be run at speeds higher than thosetheoretically possible in a bearing assembly having a rigidly mountedshaft. Also the flexible mounting of the shaft 2 prevents the onset ofhalf speed whirl of the bearing sleeve by absorption of energy from thebearing vibration and dissipation of the energy through damping in therubber ring 13 of the flexible mounting 3. The third mode of freedom ofthe shaft 2 is in the longitudinal direction which provides for thedamping of longitudinal oscillations of the bearing sleeve 5 on theshaft 2.

Also in the bearing assembly of FIG. 1, the friction drive belt 1) actson the outer surface of the bearing sleeve 5 at a greater radius thatthe radius of the bearing surfaces on the shaft 2 and in the lining 8 ofthe bearing sleeve 5. This means that the coefficient of frictionbetween the drive belt 19 and the bearing sleeve 5 necessary in order toachieve drive of the bearing sleeve can be less than the coefficient offriction between the bearing surfaces. This enhances the ability of thebearing to start up from rest when maximum friction between the bearingsurfaces exists. In addition the stationary shaft 2 has a low transversemoment of inertia so that it may readily follow any oscillations set upby the rotating bearing sleeve 5 either by resonance or out of balanceforces and therefore reduces the loading imposed on the air film in theinterspace between the surface of the shaft 2 and the bearing surface inthe plastic lining of the bearing sleeve 5 consequently allowing largeramplitudes of oscillation to be tolerated.

The above described bearing assemblies are capable of operating at veryhigh speeds and in order to do so, it is preferable that both therotatable and stationary members have low transverse moments of inertia.In particular in the bearing assemblies having a stationary shaft aroundwhich a sleeve rotates it is preferable that the stationary shaft has alow moment of inertia. The friction drive applied to the rotatablemembers in the bearing assemblies may be by either a belt or wheel.

The flexible mountings may comprise any suitable elastomeric material inaddition to soft rubber.

Referring to FIGS. 2 and 3 of the drawings a pulley assembly I is shownin which a jockey pulley 2 is located in a support member 3 which isattached to a structural member 4. A steel shaft 5 is mounted on thesupport member by a washer 6 and a nut 7. The shaft member 5 issurrounded by a close fitting aluminum sleeve member 8 having a blindend to provide a trapped volume 9 between the blind end of the sleevemember 8 and the end of the shaft 5. The sleeve 8 has a plastic lining10 and a port 11 adjacent to the blind end of the sleeve 8. The outersurface of the sleeve member 8 is provided with two annular flanges 12which are integral with the sleeve 8 on the shaft 5 and provide locationfor a driving belt 13 on the pulley 2. The shaft 5 has a precisionground outer surface and the plastic lining 10 of the sleeve 8 is formedwith a surface of gas bearing quality complementary to the shaft 5. Theplastic lining 10 of the sleeve 8 may be formed for example by themethod disclosed in our British Patent No. 979,731.

The shaft 5 is provided with a longitudinal internal air duct 14, theend of which leads into the trapped volume 9 and is sealed by a ball endstop 15. An air feed pipe 16 is connected with the other end of the airduct 14. A feed jet 17 of reduced diameter immediately below the ballend stop connects the air duct 14 with the trapped volume 9. Opposed tothe blind end of the sleeve 18 is an adjustable end stop 18 located inthe support member 3.

In operation of the arrangement of FIGS. 2 and 3 the sleeve 8 rotates onthe shaft 5. The liner 10 of the sleeve 9 acts as a hydrodynamic airlubricated bearing on the shaft 5, journal loads on the sleeve beingsupported by the pressurised cushion of air generated in the gap betweenthe sleeve 8 and the shaft 5 by rotation of the sleeve 8 on the shaft 5.

Compressed air'is fed into the trapped volume 9 at the blind end of thesleeve member 8 through the air duct 14 and the feed jet 17. Thepressure of air built up in the trapped volume 9 provides a hydrostaticair bearing supporting the sleeve member 8 longitudinally on the shaft5. The compressed air is air fed at an inlet pressure in excess of thatrequired to balance the normal downwards end loading acting on thesleeve 8 in the direction of the arrow A. Under normal end loadingconditions the sleeve 8 assumes its normal operating attitude on theshaft 5 as shown in FIG. 3 i.e., with the port 11 just covered by theend of the shaft 5. In this condition the air pressure in the trappedvolume 9 automatically adjusts so that the product of the resultantpressure and the projected area of the blind end of the sleeve 8 resultsin a total force sufficient to support the applied end load. Transientvariations in end loading acting on the sleeve 8 are accommodated bylongitudinal movement of the sleeve 8 on the shaft 5. For example atransient additional downwards end loading on the sleeve 8 forces thesleeve 8 down on to the shaft 5 in the direction of the arrow A. Theport 11 in the sleeve 8 is totally covered by the shaft 5, thus reducingthe gas flow out of the trapped volume 9 and because the feed jet 17 isof restricted corss-section the pressure drop over the feed jet 17 isreduced and the pressure of air within the trapped volume 9 adjusts tothat of the air duct 14 to counter-balance the additional downwardsloading on the sleeve 8. On return to normal end loading conditions thesleeve 8 returns to its normal 0p-- erational attitude on the shaft 5.If the sleeve 8 lifts upwards on the shaft 5 the port 11 in the sleeve 8is uncovered by the shaft 5 and the air flow from the trapped volume 9is increased, the pressure drop over the feed jet 17 is increased andthe pressure of air in the trapped volume 9 adjusts to that of theambient external pressure so that the supporting loading applied by thepressure of the air in the trapped volume 9 falls and the sleeve 8 dropsback to assume its normal operational attitude on the shaft 5.

During normal operation the above operations occur with negligible endmovement and there is negligible loss of air from the trapped volume 9because of the sizing of the feed jet 17 in the shaft 5 and the veryhigh restriction to flow in the gap between the sleeve 8 and the shaft5. Any rapid fluctuations in the position of the sleeve 8 in relation tothe shaft 5 due to external mechanical or internal pneumatic causes arecontrolled by the two and stops l5 and 18. The external stop 18 isadjustable so that minimum clearance is allowed between it and the blindend of the sleeve 8 when the position of the port 11 relative to theshaft 5 adopts its normal operating position.

An alternative form of the invention is shown in FIG. 4 in which the airpressure generated in a hydrodynamic sleeve bearing is fed to supply theair pressure for a hydrostatic air bearing providing axial support forthe sleeve.

The arrangement shown in FIG. 4 is basically similar to the arrangementshown in FIG. 3 and comprises a pulley assembly 19 in which a pulley 20is located in a support member 21. A steel shaft 22 is mounted on thesupport member 21 by a washer 24 and a nut 25. The shaft 22 issurrounded by a close fitting sleeve 26 having a blind end providing atrapped volume 27 between the blind end of the sleeve 26 and the end ofthe shaft 22. The sleeve 26 is of aluminium and has a plastic lining 28,and two diametrically opposed vent ports 29 and 30 in the region of thetrapped volume 27. The outer surface of the sleeve 26 is provided withtwo annular flanges 31 which are integral with the sleeve 26 and providelocation for a driving belt 32. The shaft 22 has a precision groundouter surface and is provided with a longitudinally located air duct 33.The end of duct 33 leading into the trapped volume 27 being sealed by aball end stop 34. A radial duct 35 located mid way along the shaft 22connects the air duct 33 to the outer surface of the shaft 22.Immediately behind the ball end stop 34 a feed jet 36 connects the airduct 33 into the trapped volume 27. Opposed to the blind end of thesleeve 26 is an adjustable end stop 37 located in the support member 21.

In operation of the arrangement of FIG. 4 the sleeve member 26 rotateson the shaft 22 as a hydrodynamic air lubricated bearing supporting thejournal loading on the sleeve 26. The air pressure in the hydrodynamicbearing is at a maximum approximately mid way along the shaft 22. Thisgenerated air pressure is fed from the surface of the shaft 22 into theair duct 33 by the duct 35. From the air duct 33 the air feed jet 36leads air from the duct into the trapped volume 27 to provide ahydrostatic bearing to support the end thrust loads on the bearing. Thenormal axial attitude of the sleeve 26 is as shown in FIG. 4 and as inthe arrangement of FIG. 3 the axial position of the sleeve 25 on theshaft 22 is controlled by uncovering of the ports 29 and 30 with axialmovements of the sleeve 26. The air pressure in the hydrostatic bearingautomatically adjusts to the predetermined value which will maintain thesleeve 26 in the position related to normal end loading of the sleeve26. The end stops 34 and 37 control any rapid fluctuations in theposition of the sleeve 26 due to mechanical or internal pneumaticcauses. The external stop 37 is adjustable so that minimum clearance isallowed between it and the blind end of the sleeve 26 when the ports 29adopt their normal operating position relative to the shaft 22. FIG.shows a modification of the embodiment shown in FIG. 4 in which theouter surface of the shaft 22 is provided with machined grooves 38. Thegrooves extend from the fixed end of the shaft 22 up to a duct 39, andare arranged so that as the sleeve 26 rotates around the shaft 22 air isdrawn along the grooves into the hydrodynamic bearing clearance betweenthe shaft 22 and the sleeve 26. This pumping action of the grooves 38leads to increased air pressure being generated in the hydronamicbearing between the shaft 22 and the sleeve 26 and therefore willprovide a greater air flow into the trapped volume 27 to sustain thehydrostatic bearing supporting the end thrusts in the pully assembly 19.

In alternative arrangements of the pulley assemblies the shafts mayrotate relative to the bearing sleeves. In addition gas pumping groovesmay be provided in the plastic lining of the bearing sleeves.

Referring to FIG. 6 of the drawings a bearing assembly l is shown inwhich a hardened steel shaft 2 is supported by a flexible mounting 3 ina structural member 4. The shaft 2 is surrounded by a close fittingbearing sleeve member 5 rotatable on the shaft 2. The bearing sleeve 5has a blind ended bore which provides a trapped volume 7 between theblind end of the bearing sleeve 5 and the end of the shaft 2. Thetrapped volume 7 is of greater diameter than the bore 6 in order toaccommodate an integral flange 8 on the end of the shaft 2. The flange 8has a lower precision ground annular bearing surface 9 which is opposedby the face of an internal step 10 in the bore 6 of the bearing sleeve5. The shaft 2 has a precision the shaft outer surface and the bore 6 ofthe bearing sleeve 5 has a plastic lining 11 formed with a surface ofgas bearing quality complementary to the surface of the shaft 12. Thebearing surface in the plastic lining ll of the bearing sleeves may beformed for example by the method disclosed in our British Patent No.979,731. The face of the internal step 10 in the bore 6 of the bearingsleeve 5 has a plastic coating 12 with a surface of gas bearing qualitycomplementary to the surface 9 on the flange 8 of theshaft 2. Thebearing surface of the plastic coating 12 is grooved to provide the gaspumping action between the bearing surfaces. Alternatively the metalbearing surface 9 may be grooved to achieve the same effect.

The shaft 2 is provided with a longitudinal internal air duct 13, theend of which leads into the trapped volume 7 and is sealed by a ballend-stop 14. A feed jet of reduced diameter and located immediatelybelow the ball end-stop 14 connects the air duct 13 into the trappedvolume 7.

In operation of the bearing assbemly 1 shown in FIG. 6 the sleeve member5 rotates on the shaft 2. The plastic lining ll of the bearing sleeve 5acts with the shaft 2 as a hydrodynamic air lubricated journal bearing,journal loads on the sleeve being supported by the pressurised cushionof air generated in the gap between the plastic lining ll of the bearingsleeve 5 and the shaft 2 by rotation of the bearing sleeve 5 on theshaft 2. Compressed air is fed into the trapped volume 7 through the airduct 13 and the feed jet 15. The pressure ofthc air built up in thetrapped volume 7 provides a hydrostatic air bearing cushion supportingthe hearing sleeve 5 longitudinally on the shaft 2 against end thrustsacting on the bearing sleeve 5 in the direction towards the shaft 2. Thepressure of the air acting in the trapped volume 7 causes the bearingsleeve 5 to take up a position on the shaft 2 such that a smallclearance exists between the bearing surface 9 on the flange 8 of theshaft 2 and the bearing surface of the plastic coat ing 12 on the step10 in the bearing sleeve 5. The bearing surface 9 on the flange 8 of theshaft 2 and the bearing surface of the plastic coating 12 on the step 10of the bearing sleeve 5 thus co-operate to act as a hydrodynamic airlubricated thrust bearing working counter to the longitudinal end thrustacting on the bearing sleeve 5 due to the pressure of the air in thetrapped volume 7. The bearing assembly 1 can thus operate in anyattitude and at the same time maintain precise location of the sleevemember 5 on the shaft 2. Ifa failure of the compressed air supply to thebearing assembly 1 should occur the ball end stop 14 ensures that a saferunning condition can be maintained for a reasonable period of time.

The arrangement shown in FIG. 7 is similar to the arrangement shown inflG. 6 and comprises a bearing assembly 16 in which a shaft 17 issupported by a flexible mounting 18 in a structural member 19. The shaft17 is surrounded by a close fitting bearing sleeve member 20 which isrotatable on the shaft 17. The bearing sleeve 20 has a blind ended bore21 which provides a trapped volume 22 between the blind end of thesleeve 20 and the end of the shaft 17. The trapped volume 22 is ofgreater diameter than the bore 21 in order to accommodate an integralflange 23 on the end of the shaft 17. The flange 23 has a lowerprecision ground annular bearing surface 24 which is opposed by the faceof an internal step 25 in the bore 21 of the bearing sleeve 20. Theshaft 17 has a precision ground outer surface and the bore 21 of thebearing sleeve 20 has a plastic lining 24 formed with a surface of gasbearing quality complementary to the surface of the shaft 17. The faceof the internal step 25 in the bore 21 of the bearing sleeve 20 has aplastic coating 27 with a surface of gas bearing quality complementaryto the bearing surface 24 on the flange 23 of the shaft 17. The surfaceof the shaft 17 is provided with machined pumping grooves 28 extendingfrom the fixed end of the shaft 17 up to radial ducts 29 located mid wayalong the shaft 17. The ducts 29 extend from the surface of the shaft 17to a longitudinal duct 30 in the shaft 17. The duct 30 leads from theducts 29 to the end of the shaft 17 in the trapped volume 22, the end ofthe duct 30 being sealed by a ball end stop 31. Immediately behind theball end stop 31 a restricted feed jet 32 connects the duct 30 with thetrapped volume 22. The surface of the plastic coating 27 on the face ofthe internal step 25 in the bore 21 of the bearing sleeve 20 is groovedto provide gas pumping action between the bearing surfaces.Alternatively the metal bearing surface 24 may be grooved to achieve thesame effect.

In operation of the bearing assembly 16 shown in FIG. 7 the sleve member20 rotates on the shaft 17 as a hydrodynamic air lubricated journalbearing, journal loads on the sleeve 20 being supported by thepressurised cushion of air generated in the gap between the plasticlining 26 of the sleeve 20 and the bearing surface of the shaft 17 byrotation of the sleeve member on the shaft 17 and by the action of thepumping grooves 28 on the shaft 17. The air pressure in the hydrodynamicjournal bearing is at a maximum approximately mid way along the shaft17. Thus compressed air is fed from the surface of the shaft 17 into theduct by the ducts 29. From the duct 30 the air feed jet 32 feeds thecompressed air into the trapped volume 22 to provide a hydrostatic airbearing cushion supporting the bearing sleeve 20 longitudinally on theshaft 17 against end thrusts acting on the bearing sleeve 20 in thedirection towards the shaft 17. The pressure of the air acting in thetrapped volume 22 causes the bearing sleeve 20 to take up a position onthe shaft 17 such that a small clearance exists between the bearingsurface 24 on the flange 23 of the shaft 17 and the bearing surface ofthe plastic coating 27 on the face of the internal step 25 in the bore21 of the bearing sleeve 20. T he bearing surface 24 and the bearingsurface of the plastic coating 27 thus co-operate to act as ahydrodynamic air lubricated thrust bearing working counter to thelongitudinal thrust acting on the bearing sleeve 20 due to the pressureof the air in the trapped volume 27. 1f the air supply tothe trappedvolume 22 is curtailed in any way the ball end stop 31 ensures that saferunning conditions can be maintained for a reasonable period of time.

FIG. 8 shows a bearing assembly 33 comprising a bearing sleeve member 34having a blind ended bore 35. The bearing sleeve 34 has an axialextension 36 by means of which the bearing sleeve 34 is supported by aflexible mounting 37 from a structural member 38. A shaft 39 isrotatable within the bore of the bearing sleeve 34. The blind ended bore35 of the bearing sleeve 34 provides a trapped volume 40 between itsblind end and the end of the shaft 39. The trapped volume 40 is ofgreater diameter than the bore 35 in order to accommodate an integralflange 41 on the end of the shaft 39. The flange 41 has an upperprecision ground annular bearing surface 42 which is opposed by the faceof an annular internal step 43 in the bore 35 of the bearing sleeve 34.The shaft 39 has a precision ground outer surface and the bore 35 of thebearing sleeve 34 has a plastic lining 44 formed with a bearing surfaceof gas bearing quality complementary to the surface of the shaft 39. Theface of the internal step 43 in the bore 35 of the bearing sleeve 34 hasa plastic coating 45 with a surface of gas bearing quality complementaryto the bearing surface 42 on the flange 41 of the shaft 39. The bearingsurface of the plastic coating 45 or the complementary bearing surface42 is grooved'to provide the gas pumping action between these bearingsurfaces when the shaft 39 is rotated.

The extension 36 of the sleeve member 34 is provided with an internalair duct 46, the end of which leads into the trapped volume 40 and issealed by a ball end-stop 47. A feed jet 48 of reduced diameter andlocated immediately below the ball end stop 47 connects the air duct 46into the trapped volume 40.

In operation of the bearing assembly 33 shown in Fig. 8, the shaft 39rotates within the stationary sleeve 34. The complementary gas bearingsurfaces of the plastic lining 44 and the shaft 39 act as a hydrodynamicair lubricated journal bearing supporting journal loads with thepressurised cushion of air generated in the gap between the bearingsurfaces. Compressed air is fed into the trapped volume 40 through theair duct 46 and the feed jet 48. The air pressure built up in thetrapped volume 40 provides a hydrostatic air bearing cushion supportingthe shaft 39 against downwards end thrusts in the bearing sleeve 34. Thepressure of the air acting in the trapped volume 40 causes the shaft 39to take up a position in the bearing sleeve 34 such that a smallclearance exists between the bearing surface 42 on the flange 41 of theshaft 39 and the bearing surface of the plastic coating 45 on the faceof the internal step 43 in the bore 35 of the bearing sleeve 34. Thebearing surface 42 and the bearing surface of the plastic coating 45thus co-operate to act as a hydrodynamic air lubricated thrust bearingworking counter to the longitudinal thrust acting on the bearing sleeve34 due to the pressure of the air in the trapped volume 40. The bearingassembly 33 can thus operate in any required attitude and at the sametime maintain precise longitudinal location of the shaft 39 in thesleeve member 34. If a failure of the compressed air supply to thebearing assembly 33 occurs the ball end stop 47 ensures that a saferunning condition can be maintained for a reasonable period of time.

The bearing assembly 49 shown in FIG. 9 is similar to that shown in FIG.8 and comprises a bearing sleeve member 50 having a blind ended bore 51.The bearing sleeve 50 has an axial extension 52 by means of which thebearing sleeve 50 is supported by a flexible mounting 53 from astructural member 54. A shaft 55 is rotatable within the bore 51 of thebearing sleeve 50. The blind ended bore 51 of the bearing sleeve 50provides a trapped volume 56. The trapped volume 56 is of greaterdiameter than the bore 51 in order to accommodate an integral flange 57on the end of the shaft 55. The flange 57 has an upper precision groundannular bearing surface 58 which is opposed by the face of an annularinternal step 59 in the bore 51 of the bearing sleeve 51. The shaft 55has a precision ground outer surface and the bore 51 of the bearingsleeve 50 has a plastic lining 60 formed with a bearing surface of gasbearing quality complementary to the surface of the shaft 55. The faceof the internal step 59 in the bore 51 of the bearing sleeve 50 has aplasticcoating 61 with a surface of gas bearing quality complementary tothe bearing surface 58 on the flange 57 of the shaft 55. The bearingsurface of the plastic coating 61 is grooved to provide gas pumpingaction between these bearing surfaces when the shaft 55 is rotated. Thesurface of the shaft is provided with machined pumping grooves 62extending from the open end of the bearing sleeve 50 up to radial ducts63 located mid way along the shaft 55 and extending from the surface ofthe shaft into a longitudinal duct 64 in the shaft 55. The duct 64 leadsto the end of the shaft 55 and is sealed at its end. lmmediatelyopposite the sealed end of the duct 64 a ball end stop 65 is locatedin'the bearing sleeve 50. A feed jet 66 connects the duct 64 into thetrapped volume 56.

I In operation of the bearing assembly 49 shown in FUlG. 9 the shaft55rotates in the fixed bearing sleeve 50, hydrodynamic air lubricationbeing sustained between the bearing surface of the shaft 55 and thebearing surface of the plastic lining 60 of the bearing sleeve 50.Journal loads on the shaft 55 are supported by the pressurised cushionof air generated in the gap between the shaft 55 and the plastic lining60 of the bearing sleeve 50. The pressure generated is due to rotationof the shaft 55 in the bearing sleeve 50 and due to the pumping actionof the grooves 62 on the surface of the shaft 55. Maximum pressure isgenerated about. mid

way along the shaft 55 in the region of the radial ducts 63 in the shaft55. Thus compressed air is fed through the ducts 63 and the longitudinalduct 64 in the shaft 55 is fed from the duct 64 through the air feed jet66 into the trapped volume 56 at the blind end of the bore 51 in thebearing sleeve 50. This provides a hydrostatic air bearing cushion inthe trapped volume 56 supporting the shaft 55 longitudinally againstdownwards end thrusts in the bearing sleeve 50. The pressure of the airacting in the trapped volume 56 causes the shaft 55 to take up aposition in the bearing sleeve 50 such that a small clearance existsbetween the bearing surface 58 on the flange 57 of the shaft 55 and thebearing surface of the plastic coating 61 on the face of the internalstop 59 in the bearing sleeve 50. The bearing surface 58 and the bearingsurface of the plastic coating 61 thus cooperate to act as ahydrodynamic air lubricated thrust bearing working counter to thelongitudinal thrust acting on the shaft 55 due to the pressure of theair into the trapped volume 56. If the air supply to the trapped volume56 is interrupted to the ball end stop 65 ensures that safe runningconditions can be maintained for a reasonable period of time.

I claim:

1. A journal gas bearing assembly comprising a shaft and a complementarybearing sleeve rotatable one relative to the other, the shaft and thebearing sleeve having co-operating bearing surfaces of a quality suchthat gas lubrication can be sustained between the shaft and the bearingsleeve, longitudinal movement of the shaft and the bearing sleeve onerelative to the other in one direction being limited by a hydrostaticthrust bearing formed by a cushion of gas trapped under pressure in aclosed volume defined between the shaft and the bearing sleeve, andpassage means leading directly into the closed volume for conveyingcompressed gas to the hydrostatic thrust bearing.

2. A journal gas bearing assembly as claimed in claim 1 wherein meansare provided for limiting longitudinal movement of the shaft and thebearing sleeve one relative to the other in the reverse direction to thedirection in which the hydrostatic thrust bearing limits relativelongitudinal movement between the shaft and the bearing sleeve.

3. A journal gas bearing assembly as claimed in claim 2, wherein saidmeans for limiting longitudinal movement of the shaft and the bearingsleeve one relative to the other in the reverse direction comprisesmeans for venting the cushion of gas in the hydrostatic thrust hearingat the beginning of relative longitudinal movement between the shaft andthe bearing sleeve in the reverse direction.

4. A journal gas bearing assembly as claimed in claim 2 wherein saidmeans for limiting longitudinal movement of the shaft and the bearingsleeve one relative to the other in the reverse direction comprises ahydrodynamic thrust gas bearing acting between the shaft and the bearingsleeve in the opposite sense to which the hydrostatic thrust bearingacts between the shaft and the bearing sleeve.

5. A gas bearing assembly as claimed in claim 1 wherein said means forfeeding compressed gas to the hydrostatic thrust bearing comprises anexternal source of compressed gas and means for supplying compressed gasfrom the source to the hydrostatic thrust bearing.

6. A gas bearing assembly as claimed in claim 1 wherein said means forfeeding compressed gas to the hydrostatic thrust bearing comprises meansfor supply ing compressed gas from the interspace between the bearingsurfaces of the shaft and the bearing sleeve to the hydrostatic thrustbearing.

7. A journal gas bearing assembly comprising a fixed shaft a bearingsleeve rotatable on the shaft the surface of the shaft and the bore ofthe bearing sleeve having co-operating bearing surfaces of a qualitysuch that gas lubrication can be sustained between the shaft and thebearing sleeve wherein the bearing sleeve is supported againstlongitudinal movement on the shaft in one direction by a hydrostaticthrust bearing formed by a cushion of gas trapped in a closed volumedefined in the bore of the bearing sleeve between a transverse face onthe shaft and a transverse face in the bore of the bearing sleeve, andpassage means leading directly into the closed volume for conveyingcompressed gas to the hyddrostatic thrust bearing.

8. A journal gas bearing assembly as claimed in claim 7 wherein thebearing sleeve has a blind ended bore and the closed volume is definedbetween the blind end of the bore of the bearing sleeve and the end faceof the shaft in the bore of the bearing sleeve.

9. A journal gas bearing assembly as claimed in claim 7 wherein themeans for feeding compressed gas to the hydrostatic thrust bearingcomprises a passageway in the shaft leading to the closed volume definedin the bore of the bearing sleeve, compressed gas being fed from anexternal source of gas under pressure through the passageway in theshaft into the closed volume defined in the bore of the bearing sleeve.

10. A journal gas bearing assembly as claimed in claim 7 wherein saidmeans for feeding compressed gas to the hydrostatic thrust bearingcomprises a passageway in the shaft leading from the bearing interspacebetween the surface of the shaft and the bore of the bearing sleeve tothe closed volume defined in the bore of the bearing sleeve.

11. A journal gas bearing assembly as claimed in claim 10 whereinpumping grooves are provided on a region of the surface of the shaft orin the corresponding region of the bore of the bearing sleeve, saidpassageway leading from the region of the pumping grooves to the closedvolume defined in the bore of the bearing sleeve.

12. A journal gas bearing assembly as claimed in claim 9 wherein thepassageway connects with the closed volume defined in the bore of thebearing sleeve through an orifice of restricted cross section.

13. A journal gas bearing assembly as claimed in claim 7 wherein meansare provided for limiting longitudinal movement of the bearing sleeve onthe shaft in the opposite direction to the direction in which thebearing sleeve is supported against longitudinal movement on the shaftby the hydrostatic thrust bearing.

14. A journal gas bearing assembly as claimed in claim 13 wherein a venthole is provided in the wall of the bearing sleeve positioned so as tobe obstructed by the surface of the shaft when the bearing sleeve is inits normal running position on the shaft supported against longitudinalmovement, in the one direction, on the shaft by the hydrostatic thrustbearing and such that on initial longitudinal movement of the bearingsleeve on the shaft in the opposite direction, the vent hole in thebearing sleeve is uncovered by the shaft so that the closed volume inthe bore of the bearing sleeve defining the hydrostatic thrust bearingis vented to atmosphere.

15. A journal gas bearing assembly as claimed in claim 13 wherein ahydrodynamic thrust bearing is provided defined between a transferseannular bearing surface of the bearing sleeve and a correspondingtransverse annular gearing surface on the shaft, said hydrodynamicthrust bearing supporting the bearing sleeve longitudinally on the shaftin the opposite direction to which the hydrostatic thrust bearingsupports the bearing sleeve longitudinally on the shaft.

16. A journal gas bearing assembly as claimed in claim 15 wherein thebore of the bearing sleeve has a blind end and leads through a step intoa part of larger diameter at the blind end of the bore, the annular endface of the step forming one bearing surface of the hydrodynamic thrustbearing, the shaft having an end flange in the part of larger diameterat the blind end of the bore of the bearing sleeve, the annular end faceof the flange facing the annular end face of the step in the bore of thebearing sleeve forming the other bearing surface of the hydrodynamicthrust bearing.

17. A journal gas bearing assembly as claimed in claim 7 wherein theshaft is flexibly mounted and has a low transverse moment of inertia.

18. A journal gas bearing assembly as claimed in claim 7 wherein thebearing sleeve is driven by a friction drive member engaging the bearingsleeve tangentially within the region between the ends of the shaft.

19. A journal gas bearing assembly comprising a fixed bearing sleeve, ashaft rotatable in the bearing sleeve, the surface of the shaft and thebore of the bearing sleeve having cooperating bearing surfaces of aquality such that gas lubrication can be sustained between the shaft andthe bearing sleeve wherein the shaft is supported against longitudinalmovement in the bearing sleeve in one direction by a hydrostatic thrustbearing formed by a cushion of gas trapped in a closed colume defined inthe bore of the bearing sleeve between a transverse face on the shaftand a transferse face in the bore of the bearing sleeve, and passagemeans leading directly into the closed volume for conveying compressedgas to the hydrostatic thrust bearmg.

20. A journal gas bearing assembly as claiemd in claim 19 wherein thebearing sleeve has a blind ended bore and the closed volume is definedbetween the blind end of the bore of the bearing sleeve and the end faceof the shaft in the bore of the bearing sleeve.

21. A journal gas bearing as claimed in claim 19 wherein the means forfeeding compressed gas to the hydrostatic thrust bearing comprises aninlet port in the bearing sleeve connecting an external source of gasunder pressure with the closed volume defined in the bore of the bearingsleeve.

22. A journal gas bearing assembly as claimed in claim 19 wherein saidmeans for feeding compressed gas to the hydrostatic thrust bearingcomprises a passageway in the shaft leading from the bearing interspacebetween the surface of the shaft and the bore of the bearing sleeve tothe closed volume defined in the bore of the bearing sleeve.

23. A journal gas bearing assembly as claimed in claim 22 whereinpumping grooves are provided on a region of the surface of the shaft orin the corresponding region of the bore of the bearing sleeve, saidpassageway leading from the region of the pumping grooves to the closedvolume defined in the bore of the bearing sleeve.

24. A journal gas bearing assembly as claimed in claim 22 wherein thepassageway connects with the closed volume defined in the bore of thebearing sleeve through an orifice of restricted cross section.

25. A journal gas bearing assembly as claimed in claim 19 wherein meansare provided for limiting longitudinal movement of the shaft in thebearing sleeve in the opposite direction to the direction in which theshaft is supported against longitudinal movement in the bearing sleeveby the hydrostatic thrust bearing.

26. A journal gas bearing assembly as claimed in claim 25 wherein a venthole is provided in the wall of the bearing sleeve, positioned so as tobe obstructed by the surface of the shaft when it is in its normalrunning position in the bearing sleeve, supported against longitudinalmovement, in the one direction, in the bearing sleeve by the hydrostaticthrust bearing and such that on initial longitudinal movement of theshaft in the bearing sleeve in the opposite direction the vent hole inthe bearing sleeve is uncovered by the shaft so that the closed volumein the bore of the bearing sleeve defining the hydrostatic thrustbearing is vented to atmosphere.

27. A journal gas bearing assembly as claimed in claim 25 wherein ahydroynamic thrust bearing is provided defined between a transverseannular bearing surface of the bearing sleeve and a correspondingtransverse annular bearing surface on the shaft, said hydrodynamicthrust bearing supporting the shaft longitudinally in the bearing sleevein the opposite direction to which the hydrostatic thrust bearingsupports the shaft longitudinally in the bearing sleeve.

28. A journal gas bearing assembly as claimed in claim 27 wherein thebore of the bearing sleeve has a blind end and leads through a step intoa part of larger diameter at the blind end of the bore, the annular endface of the step forming one bearing surface of the hydrodynamic thrustbearing, the shaft having an end flange in the part of larger diameterat the blind end of the bore of the bearing sleeve, the annular end faceof the flange facing the annular end face of the step in the bore of thebearing sleeve forming the other bearing surface of the hydrodynamicthrust bearing.

29. A journal gas bearing assembly as claimed in claim 19 wherein thebearing sleeve is flexibly mounted and has a low transverse moment ofinertia.

30. Ajoumal gas varying assembly as claimed in claim 10 wherein thepassageway connects with the closed volume defined in the bore of thebearing sleeve through an orificeof restricted cross section.

31. A journal gas bearing assembly as claimed in claim 23 wherein thepassageway connects with the closed volume defined in the bore of thebearing sleeve through an orifice of restricted cross section.

t I? i i 1'

1. A journal gas bearing assembly comprising a shaft and a complementarybearing sleeve rotatable one relative to the other, the shaft and thebearing sleeve having co-operating bearing surfaces of a quality suchthat gas lubrication can be sustained between the shaft and the bearingsleeve, longitudinal movement of the shaft and the bearing sleeve onerelative to the other in one direction being limited by a hydrostaticthrust bearing formed by a cushion of gas trapped under pressure in aclosed volume defined between the shaft and the bearing sleeve, andpassage means leading directly into the closed volume for conveyingcompressed gas to the hydrostatic thrust bearing.
 2. A journal gasbearing assembly as claimed in claim 1 wherein means are provided forlimiting longitudinal movement of the shaft and the bearing sleeve onerelative to the other in the reverse direction to the direction in whichthe hydrostatic thrust bearing limits relative longitudinal movementbetween the shaft and the bearing sleeve.
 3. A journal gas bearingassembly as claimed in claim 2, wherein said means for limitinglongitudinal movement of the shaft and the bearing sleeve one relativeto the other in the reverse direction comprises means for venting thecushion of gas in the hydrostatic thrust bearing at the beginning ofrelative longitudinal movement between the shaft and the bearing sleevein the reverse direction.
 4. A journal gas bearing assembly as claimedin claim 2 wherein said means for limiting longitudinal movement of theshaft and the bearing sleeve one relative to the other in the reversedirection comprises a hydrodynamic thrust gas bearing acting between theshaft and the bearing sleeve in the opposite sense to which thehydrostatic thrust bearing acts between the shaft and the bearingsleeve.
 5. A gas bearing assembly as claimed in claim 1 wherein saidmeans for feeding compressed gas to the hydrostatic thrust bearingcomprises an external source of compressed gas and means for supplyingcompressed gas from the source to the hydrostatic thrust bearing.
 6. Agas bearing assembly as claimed in claim 1 wherein said means forfeeding compRessed gas to the hydrostatic thrust bearing comprises meansfor supplying compressed gas from the interspace between the bearingsurfaces of the shaft and the bearing sleeve to the hydrostatic thrustbearing.
 7. A journal gas bearing assembly comprising a fixed shaft abearing sleeve rotatable on the shaft the surface of the shaft and thebore of the bearing sleeve having co-operating bearing surfaces of aquality such that gas lubrication can be sustained between the shaft andthe bearing sleeve wherein the bearing sleeve is supported againstlongitudinal movement on the shaft in one direction by a hydrostaticthrust bearing formed by a cushion of gas trapped in a closed volumedefined in the bore of the bearing sleeve between a transverse face onthe shaft and a transverse face in the bore of the bearing sleeve, andpassage means leading directly into the closed volume for conveyingcompressed gas to the hyddrostatic thrust bearing.
 8. A journal gasbearing assembly as claimed in claim 7 wherein the bearing sleeve has ablind ended bore and the closed volume is defined between the blind endof the bore of the bearing sleeve and the end face of the shaft in thebore of the bearing sleeve.
 9. A journal gas bearing assembly as claimedin claim 7 wherein the means for feeding compressed gas to thehydrostatic thrust bearing comprises a passageway in the shaft leadingto the closed volume defined in the bore of the bearing sleeve,compressed gas being fed from an external source of gas under pressurethrough the passageway in the shaft into the closed volume defined inthe bore of the bearing sleeve.
 10. A journal gas bearing assembly asclaimed in claim 7 wherein said means for feeding compressed gas to thehydrostatic thrust bearing comprises a passageway in the shaft leadingfrom the bearing interspace between the surface of the shaft and thebore of the bearing sleeve to the closed volume defined in the bore ofthe bearing sleeve.
 11. A journal gas bearing assembly as claimed inclaim 10 wherein pumping grooves are provided on a region of the surfaceof the shaft or in the corresponding region of the bore of the bearingsleeve, said passageway leading from the region of the pumping groovesto the closed volume defined in the bore of the bearing sleeve.
 12. Ajournal gas bearing assembly as claimed in claim 9 wherein thepassageway connects with the closed volume defined in the bore of thebearing sleeve through an orifice of restricted cross section.
 13. Ajournal gas bearing assembly as claimed in claim 7 wherein means areprovided for limiting longitudinal movement of the bearing sleeve on theshaft in the opposite direction to the direction in which the bearingsleeve is supported against longitudinal movement on the shaft by thehydrostatic thrust bearing.
 14. A journal gas bearing assembly asclaimed in claim 13 wherein a vent hole is provided in the wall of thebearing sleeve positioned so as to be obstructed by the surface of theshaft when the bearing sleeve is in its normal running position on theshaft supported against longitudinal movement, in the one direction, onthe shaft by the hydrostatic thrust bearing and such that on initiallongitudinal movement of the bearing sleeve on the shaft in the oppositedirection, the vent hole in the bearing sleeve is uncovered by the shaftso that the closed volume in the bore of the bearing sleeve defining thehydrostatic thrust bearing is vented to atmosphere.
 15. A journal gasbearing assembly as claimed in claim 13 wherein a hydrodynamic thrustbearing is provided defined between a transferse annular bearing surfaceof the bearing sleeve and a corresponding transverse annular gearingsurface on the shaft, said hydrodynamic thrust bearing supporting thebearing sleeve longitudinally on the shaft in the opposite direction towhich the hydrostatic thrust bearing supports the bearing sleevelongitudinally on the shaft.
 16. A journal gas bearing assembly asclaimed in claim 15 wherein the bore of the beAring sleeve has a blindend and leads through a step into a part of larger diameter at the blindend of the bore, the annular end face of the step forming one bearingsurface of the hydrodynamic thrust bearing, the shaft having an endflange in the part of larger diameter at the blind end of the bore ofthe bearing sleeve, the annular end face of the flange facing theannular end face of the step in the bore of the bearing sleeve formingthe other bearing surface of the hydrodynamic thrust bearing.
 17. Ajournal gas bearing assembly as claimed in claim 7 wherein the shaft isflexibly mounted and has a low transverse moment of inertia.
 18. Ajournal gas bearing assembly as claimed in claim 7 wherein the bearingsleeve is driven by a friction drive member engaging the bearing sleevetangentially within the region between the ends of the shaft.
 19. Ajournal gas bearing assembly comprising a fixed bearing sleeve, a shaftrotatable in the bearing sleeve, the surface of the shaft and the boreof the bearing sleeve having cooperating bearing surfaces of a qualitysuch that gas lubrication can be sustained between the shaft and thebearing sleeve wherein the shaft is supported against longitudinalmovement in the bearing sleeve in one direction by a hydrostatic thrustbearing formed by a cushion of gas trapped in a closed colume defined inthe bore of the bearing sleeve between a transverse face on the shaftand a transferse face in the bore of the bearing sleeve, and passagemeans leading directly into the closed volume for conveying compressedgas to the hydrostatic thrust bearing.
 20. A journal gas bearingassembly as claiemd in claim 19 wherein the bearing sleeve has a blindended bore and the closed volume is defined between the blind end of thebore of the bearing sleeve and the end face of the shaft in the bore ofthe bearing sleeve.
 21. A journal gas bearing as claimed in claim 19wherein the means for feeding compressed gas to the hydrostatic thrustbearing comprises an inlet port in the bearing sleeve connecting anexternal source of gas under pressure with the closed volume defined inthe bore of the bearing sleeve.
 22. A journal gas bearing assembly asclaimed in claim 19 wherein said means for feeding compressed gas to thehydrostatic thrust bearing comprises a passageway in the shaft leadingfrom the bearing interspace between the surface of the shaft and thebore of the bearing sleeve to the closed volume defined in the bore ofthe bearing sleeve.
 23. A journal gas bearing assembly as claimed inclaim 22 wherein pumping grooves are provided on a region of the surfaceof the shaft or in the corresponding region of the bore of the bearingsleeve, said passageway leading from the region of the pumping groovesto the closed volume defined in the bore of the bearing sleeve.
 24. Ajournal gas bearing assembly as claimed in claim 22 wherein thepassageway connects with the closed volume defined in the bore of thebearing sleeve through an orifice of restricted cross section.
 25. Ajournal gas bearing assembly as claimed in claim 19 wherein means areprovided for limiting longitudinal movement of the shaft in the bearingsleeve in the opposite direction to the direction in which the shaft issupported against longitudinal movement in the bearing sleeve by thehydrostatic thrust bearing.
 26. A journal gas bearing assembly asclaimed in claim 25 wherein a vent hole is provided in the wall of thebearing sleeve, positioned so as to be obstructed by the surface of theshaft when it is in its normal running position in the bearing sleeve,supported against longitudinal movement, in the one direction, in thebearing sleeve by the hydrostatic thrust bearing and such that oninitial longitudinal movement of the shaft in the bearing sleeve in theopposite direction the vent hole in the bearing sleeve is uncovered bythe shaft so that the closed volume in the bore of the bearing sleevedefining the hydrostatic thrust bearing is vented to atmosphere.
 27. Ajournal gas bearing assembly as claimed in claim 25 wherein ahydroynamic thrust bearing is provided defined between a transverseannular bearing surface of the bearing sleeve and a correspondingtransverse annular bearing surface on the shaft, said hydrodynamicthrust bearing supporting the shaft longitudinally in the bearing sleevein the opposite direction to which the hydrostatic thrust bearingsupports the shaft longitudinally in the bearing sleeve.
 28. A journalgas bearing assembly as claimed in claim 27 wherein the bore of thebearing sleeve has a blind end and leads through a step into a part oflarger diameter at the blind end of the bore, the annular end face ofthe step forming one bearing surface of the hydrodynamic thrust bearing,the shaft having an end flange in the part of larger diameter at theblind end of the bore of the bearing sleeve, the annular end face of theflange facing the annular end face of the step in the bore of thebearing sleeve forming the other bearing surface of the hydrodynamicthrust bearing.
 29. A journal gas bearing assembly as claimed in claim19 wherein the bearing sleeve is flexibly mounted and has a lowtransverse moment of inertia.
 30. Ajournal gas varying assembly asclaimed in claim 10 wherein the passageway connects with the closedvolume defined in the bore of the bearing sleeve through an orifice ofrestricted cross section.
 31. A journal gas bearing assembly as claimedin claim 23 wherein the passageway connects with the closed volumedefined in the bore of the bearing sleeve through an orifice ofrestricted cross section.